A big positional concession.
18. c1b3 The endgame is very unpleasant for Black. He doesn't have an active plan, and the d6-pawn will suffer pressure from a bishop on g3. The queenside overall is a bit shaky. 18... f8c8  19. f2g3  19... e7f8 This doesn't help Black's position, although it's difficult to recommend active moves. 20. f1e2 